2,2-Bipyridine Ligand, Sensitizing Dye and Dye Sensitized Solar Cell

ABSTRACT

A dye sensitized solar cell, comprising a-heteroleptic polypyridil complex of Ru, Os or Fe. The donating ligand has an extended conjugated n-system increasing the light absorbance and keeing the LUMO energy level higher than that of the anchoring ligand. A compacting compound whose molecular structure comprises a terminal group, a hydrophobic part and an anchoring′ group may be co-adsorbed together with the dye on the semi-conductive metal oxide layer of the photoanode, forming a dense mixed self-assembled monolayer.

This application claims the benefits under 35 U.S.C. 119(a)-(d) or (b),or (365(b) of International Application No. PCT/CH2005/000452 filed Jul.29, 2005, and European Patent Application No. 04405484.9 filed Jul. 29,2004.

The present invention concerns an organic compound. The presentinvention further concerns a sensitizing dye incorporating said organiccompound. Still further, the present invention concerns a dye-sensitizedsolar cell.

Dye-sensitized solar cells, or DSSCs, are regenerativephoto-electrochemical cells comprising a photoanode, said photoanodecomprising at least one semiconductive metal oxide layer on a conductivesubstrate, sensitized by at least one chromophoric substance, acounter-electrode, and an electrolyte positioned between theseelectrodes.

In cells of this type, at least one of these electrodes is sufficientlytransparent or translucent for allowing light input. The afore-saidsemi-conductive metal oxide layer is conveniently made of oxides oftransition metals or elements either of the third main group, or of thefourth, fifth and sixth sub-groups of periodic table of elements, thesurface of the photoanode in contact with the electrolyte being porous,with a porosity factor of preferably at least 20. The “porosity factor”is defined as the ratio of the photo-electrochemically active surface ofthe photoanode to the surface area of the substrate covered by thelayer(s) of semiconductive metal oxide. The use of nanocrystallinetitanium dioxide was shown to be particularly advantageous. The term“nanocrystalline” means that the semiconductive metal oxide, inparticular TiO₂, is in polycrystalline form with a granulometry of theorder of several nanometers, for example 10 to 50 nanometers.

In this type of cell, a chromophoric substance, often calledphotosensitizer or photosensitizing dye, forms a substantiallymonomolecular layer attached to the semiconductive metal oxide layer, inparticular the nanocrystalline TiO₂ layer. The chromophoric substancemay be bound to the metal oxide layer by means of anchoring groups likecarboxylate or phosphonate or cyano groups or chelating groups withπ-conducting character like oxymes, dioxymes, hydroxyquinolines,salicylates and (-keto-enolates. Several transition metal complexes, inparticular ruthenium complexes, but also osmium or iron complexes, withheterocyclic ligands like bidentate, tridentate or polydentatepolypyridil compounds, have been shown to be efficient photosensitizingdyes. Sensitizing dyes and cells of this type are described inter aliain EP 0333641, EP 0525070, EP 0613466 and EP 0758337.

The mesoporous texture of the TiO₂ film in these cells significantlyincreases the cross section of light harvesting by surface-anchoredcharge transfer sensitizers while maintaining a good contact withelectrolytes. In these photovoltaic devices, ultrafastelectron-injection from a photoexcited dye into the conduction band ofan oxide semiconductor, and subsequently dye regeneration and holetransportation to the counter electrode, are responsible for theefficient generation of electricity.

Among suitable electrolytes are those including a redox systemconsisting of a mixture of at least one electrochemically active saltand at least one molecule designed to form an oxidation-reduction systemwith either the anion or cation of the said salt. Electrolytes whereinsaid electrochemically active salt has a melting point below ambienttemperature or forms with the afore-said molecule a phase with a meltingpoint below ambient temperature have been described in EP 0737358.Additionally, gelified materials incorporating triiodide/iodide as aredox couple, as disclosed by EP 1087412, were introduced to substitutethe liquid electrolytes by quasi-solid state materials.

A respectable 10.4% light-to-electricity conversion efficiency at AM 1.5solar irradiance has been obtained for photovoltaic devices with apanchromatic dye and a liquid electrolyte containing thetriiodide/iodide couple, as reported in J. Am. Chem. Soc. 123, 1613-1624(2001).

However the achievement of long-term stability at temperatures of 80-85°C., which is an important requirement for outdoor application of theDSSC, still remains a major challenge:

The leakage of liquid electrolyte from such DSSC modules, possibledesorption of loosely attached dyes and photo-degradation in thedesorbed state, as well as corrosion of the photoelectrode and/orcounter electrode by the triiodide/iodide couple, may be considered assome critical factors limiting the long-term performance of the DSSC,especially at elevated temperature. A particular problem of stability at80° C. in DSSCs containing the iodide/triiodide redox couple, uponaging, is the drop in open circuit potential (V_(oc)), causing the poorstability. It is believed that the dark current of DSSCs increases andV_(oc) decreases, due to the interaction of triiodide with bare zones ofthe TiO₂ electrode, that is not completely covered with dye molecules.

Grätzel and co-workers demonstrated (Langmuir (2002) 18, 952) thatheteroleptic amphiphilic complexes of formula RuLL′(NCS)₂, where L isthe anchoring ligand 4,4′-dicarboxy-2,2′-bipyridine and L′ is a2,2′-bipyridine substituted by one or 2 long alkyl chains, are aninteresting class of sensitizing dyes for DSSCs. The long alkyl chainsin all likelihood interact laterally to form an aliphatic network,thereby impeding triiodide from reaching the TiO₂ surface, resulting inincreased open circuit potential of the cell and enhanced stabilityversus time. They further found (Nature Materials (2003) 2, 402) that acell using the sensitizer cis-(NCS)₂RuLL′, whereL′=4,4′-dynonyl-2,2′-bipyridine, hereinafter referred to as Z907, inconjunction with a quasi solid state polymer gel electrolyte reaches anefficiency of >6% in full sunlight (air mass 1.5, 100 mW·cm⁻²) withunprecedented stable performance under both thermal stress and soakingwith light.

A further remarkable increase in photovoltaic performance was achievedby co-grafting hexadecylmalonic acid (HDMA) with Z907 sensitizing dyeonto nanocrystalline TiO₂ films (J. Phys. Chem. B (2003) 107, 14336).Like Z907, HDMA contains two carboxylate groups to anchor it on the TiO₂surface. Co-grafting of the two amphiphiles results in the formation ofa mixed monolayer which should be more tightly packed than when thesensitizing dye is adsorbed alone, providing a more effective insulatingbarrier for the back electron transfer from TiO₂ conduction band totriiodide in the electrode. Retarding this unwanted redox process by thehydrophobic spacer reduces the dark current and increases the opencircuit voltage of the solar cell. The cell also showed good stabilityunder light soaking at 55° C. in simulated sunlight.

However, the molar extinction coefficient of known amphiphilicpolypyridyl ruthenium sensitizers is lower than the one of N-719, themost efficient sensitizer for dye-sensitized solar cells. Additionally,the spectral response is blue-shifted compared with this most efficientsensitizing dye.

Thus, the aim of the invention is to improve the light-harvestingcapacity of amphiphilic sensitizing dyes by a rational design of themolecule while not decreasing their LUMO energy, allowing a high quantumefficiency of electron injection without lowering the conduction band ofthe mesoporous semiconductor and thus having a loss of devicephotovoltage.

These aims are achieved by using, as a ligand, an organic compound L1having a formula selected from the group of formulae (a) to (j)

wherein at least one of substituents —R, —R₁, —R₂, —R₃,

—R′, —R₁′, —R₂′, —R₃′, —R″ comprises an additional π system located inconjugated relationship with the primary π system of the bidentate orrespectively tridentate structure of formulae (a) to (j).

Briefly speaking, use of compounds L1 permits to extend the conjugated πsystem of the donating ligand, increasing the light absorbance andkeeing the LUMO energy level higher than that of the anchoring ligand.

In preferred compounds L1, the said substituent is of the type

—R=ΠRal)_(q)

wherein Π represents schematically the π system of the aforesaidsubstituent, Ral represents an aliphatic substituent with a saturatedchain portion bound to the π system, and wherein q represents aninteger, indicating that Π may bear more than one substituent Ral.

The π system Π may be an unsaturated chain of conjugated double ortriple bonds of the type

wherein p is an integer from 1 to 8.

or an aromatic group Rar of from 6 to 22 carbon atoms, or a combinationthereof.

The presence of an aromatic group is preferred, since it is lesssensitive to oxidation than a long chain of conjugated double or triplebonds.

Among suitable aromatic groups, there are monocyclic aryls like benzeneand annulenes, oligocyclic aryls like biphenyl, naphthalene,biphenylene, azulene, phenanthrene, anthracene, tetracene, pentacene, orperylene. The cyclic structure of Rar may incorporate heteroatoms.

Preferred ligands according to the invention are organic compounds L1having a formula selected from the group of formulae (a) to (j)

-   -   wherein at least one of substituents —R, —R₁, —R₂, —R₃, —R′,        —R₁′, —R₂′, —R₃′, —R″ is of formula (1), (2) or (3)

wherein p is an integer from 1 to 4,wherein q is an integer from 1 to 4,wherein Rar is a monocyclic or oligocyclic aryl from C6 to C22,wherein —Ral is H, —R1, (—O—R1)_(n), —N(R1)₂, —NHR1,

wherein R1, R′1 is an alkyl from 1 to 10 carbon atoms, 20≧X≧0, and5≧n≧0, 8≧Y≧1, Z=1 or 2, and wherein the other one(s) of substituent(s)—R, —R₁, —R₂, —R₃, —R′, —R₁′, —R₂′, —R₃′, —R″ is (are) the same or adifferent substituent of formula (1), (2) or (3), or is (are) selectedfrom —H, —OH, —R₂, —OR₂ or —N(R₂)₂, wherein R₂ is an alkyl of 1 to 20carbon atoms.

Compounds L1, in which p=1 are preferred, because the molecularstructure is more rigid, less sensitive to oxidation, but is still anelectron donator.

The invention faces more particularly compounds L1, wherein saidcompound is a 4,4′-disubstituted bipyridine of formula

wherein p=1;

more particularly compounds L1, wherein R═R′, p=1, and wherein Rar isselected from the group consisting of benzene, naphthalene andanthracene.

Particularly preferred compounds L1 are:

-   4,4′-bis(4-hexyloxystyryl)-2,2′ bipyridine,-   4,4′-bis(4-hexyloxynaphtalene-1-vinyl)-2,2′ bipyridine,-   and 4,4′-bis[4-(1,4,7,10-Tetraoxyundecyl)styryl]-2,2′-bipyridine].

FIGS. 11 and 12 show further examples 1 to 10 of L1 compounds.

The resulting sensitizing dye is an organometallic complex of a metal Meselected from the group consisting of Ru, Os and Fe, comprising as aligand a compound L1 as described herein before, said complex being offormula

Me L1 L(Z)₂  (I)

if L1 is a compound of formula (a′), (b), (c), (d), (g), (h), (i) or (j)

and of formula

Me L1 L Z  (II)

if L1 is a compound of formula (e) or (f)

wherein L is a ligand selected from the group of ligands of formula

wherein A and A′ are anchoring groups selected from COOH, PO₃H₂, PO₄H₂,SO₃H₂, SO₄H₂, CONHOH, deprotonated forms thereof and chelating groupswith Π conducting character,

wherein Z is selected from the group consisting of H₂O, Cl, Br, CN, NCO,NCS and NCSe and

wherein at least one of substituents R, R′, R″ comprises a π system inconjugated relationship with the π system of the bidentate, respectivelythe tridentate structure of formulae (a′) to (j),

and wherein the other one(s) of substituents R, R′, R″ is (are) the sameor a different substituent including a π system, or is (are) selectedfrom H, OH, R2, (OR2)_(n), N(R2)₂, where R2 is an alkyl of 1-20 carbonatoms and 0<n<5.

More particularly, the sensitizing dye according to the invention is acomplex of formula

Me L1 L(Z)₂  (I)

wherein Me designates Ru, Os or Fe,

wherein L is selected from ligands

wherein Z is selected from H₂O, —Cl, —Br, —I, —CN, —NCO, —NCS and —NCSe.

wherein L1 is a 4,4′ disubstituted bipyridine of formula

wherein R is a substituent selected from the group of substituents (1),(2) and (3), and R′ has the same meaning as above.

wherein p is an integer from 1 to 4 or is 0wherein q is an integer from 1 to 4wherein Rar is a monocyclic or polycyclic aryl from C6 to C22wherein each —Ral is, independently one from the others, —H, —R1,—(O—R1)_(n), —NHR1, N(R1)₂,

wherein R1, R′1 is an alkyl from 1 to 10 carbon atoms, 20≧X≧0, and5≧n≧0, 8≧Y≧1, Z=1 or 2.

The use of heteroleptic ruthenium (II) sensitizing dyes may be preferredover the symmetrical ones. Heteroleptic sensitizing dyes can incorporaterequired properties in one molecule by selecting suitable ligands toenhance the photovoltaic performance.

A preferred family of sensitizers are Ru complexes of formula

cis(NCS)₂RuLL1,

wherein L1 is of formula (a′), wherein R is of formula (1), (2) or (3),wherein p=1, wherein Rar is selected from the group consisting ofbenzene, naphthalene, wherein q=1 to 4, wherein Ral is OR1 and whereinR1 is an alkyl of 1 to 10 carbon atoms.

Particularly preferred sensitizers are:

cis-dithiocyanato-(2,2′-bipyridyl-4,4′-dicarboxylate)-[4,4′-bis(4-hexyloxystyryl)-2,2′bipyridyl]-Ru(II), hereinafter referred to as K19,

cis-dithiocyanato-(2,2′-bipyridyl-4,4′-dicarboxylate)-[4,4′-bis(4-hexyloxynaphtalene-1-vinyl)-2,2′bipyridyl]-Ru(II), hereinafter referred to as K24,

Cis-dithiocyanato-(2,2′-bipyridyl-4,4′-dicarboxylate)-[4,4′-bis[4-(1,4,7,10-Tetraoxyundecyl)styryl]-2,2′-bipyridine]-Ru(II)hereinafter referred to as K60, and

cis-dithiocyanato-(2,2′-bipyridyl-4,4′-dicarboxylate)-[4,4′-bis(3-methoxystyryl)-2,2′bipyridyl]-Ru(II), hereinafter referred to as Z910.

These new sensitizing dyes have a very high light-harvesting capacity toUV photons. The UV photons can directly excite the wide band-gap metaloxide semiconductor to produce chemically active holes and thusdecompose sensitizing dyes and organic electrolyte components orhole-transport materials. After absorbing of UV photons by these newsensitizers, excitons can move rapidly from the donating ligand to themetal center, leaving a hole there and giving an electron to theanchoring ligand, and the electron will be injected to the semiconductorfilm and realize interfacial charge separation. The strong UV photonabsorbing ability makes this type of sensitizer like a “UV filter” whilehaving the advantage of converting the normally unwanted UV photons fordye sensitized solar cells to useful electrons.

These new sensitizing dyes with enhanced light-harvesting capacity areparticularly advantageous when used in combination with transparentmesoporous films (no scattering layer), and/or high-viscosity ionicliquid electrolytes, with which thinner mesoporous films are needed toreduce the mass transport problem, said thinner films having arelatively low surface area (larger metal oxide semiconductor particles)for inducing less back electron transfer. Additionally, with thisexcellent light harvesting property of these sensitizing dyes, lessamount of materials are required for efficient devices.

According to a further aspect of the DSSC according to the presentinvention, an amphiphilic compacting compound is co-adsorbed with thedye on the surface of the semiconductive metal oxide layer forming amixed monolayer. The molecular structure of said compacting compoundcomprises at least one anchoring group, a hydrophobic portion and aterminal group.

The anchoring group of the compacting compound, binding to the surfaceof the semiconductive metal oxide layer, may be the same as theanchoring group of the sensitizing dye or a different one. It may beselected from the group consisting of COOH, PO₃H₂, PO₄H₂, SO₃H₂, SO₄H₂,CONHOH or deprotonated forms thereof. The anchoring group of thecompacting compound may also be a chelating group with π-conductingcharacter, in particular an oxyme, dioxyme, hydroxyquinoline, salicylateor α-keto-enolate group.

The molar ratio of said sensitizing dye to said co-adsorbed compactingcompound may be of between 10 and ½, and preferably of between 5 and 1.Depending upon the selection of the dye and the co-adsorbent, i.e. theirrelative affinity constant for the TiO₂ layer, the ratio of dye andco-adsorbent can be varied from 1:10 to 10:1 in their common solvent ifthey are adsorbed simultaneously, i.e. within the same preparative step.Alternatively, the compacting compound may be adsorbed in a preliminaryadsorption step, before the adsorption of the dye, as a pre-treatment,or after the adsorption of the dye, as a post-treatment separateadsorption step.

Since optical density measurements of the mixed monolayer show adecrease in the optical density, if compared to the optical density ofan adsorbed monolayer made of a neat dye, it appears that the compactingagent does go onto the surface along with dye molecules, rendering sucha monolayer compact. It is thus believed that said sensitizing dye andsaid compacting compound form a self-assembled compact mixed monolayeron said semiconductive metal oxide layer.

Without being bound by theory, it is believed that the hydrophobic partof the amphiphilic sensitizing dye molecules and the hydrophobic portionof the compacting compound molecules co-adsorbed in the afore-saidratios constitute a closely packed hydrophobic monolayer forming abarrier shielding the surface of the semiconductor metal oxide layer, inparticular versus triiodide. It is believed that the triiodide can nomore reach the TiO₂ surface and that therefore the dark currentdecreases by decreasing the back electron transfer from the photoinjected electrons of TiO₂ to triiodide. It is also believed that thehydrophobic portion of the mixed monolayer constitutes a barrier againstH₂O, hindering water residues to reach the surface of the photoanode. Itis further believed that the presence of the co-adsorbing compactingcompound contributes in structuring the arrangement of the adsorbed dyemolecules.

The terminal group of the compacting compound may be an uncharged group.The terminal group may consist of the free end of an alkyl, alkenyl,alkynyl, alkoxyl or poly-ether chain. The terminal group may consist ofa neutral group taking up more space, like a branched alkyl, or a carbonatom substituted by several cycloalkyl or phenyl groups.

Without being bound by theory, it is believed that when the compactingmolecules co-adsorbed with the sensitizing dye have a sufficient chainlength and if the ends of these chains bear a terminal group (Y)constituted by a bulky neutral hydrophobic group like branched alkyls,these terminal groups have a capping function protecting the dye layerand the anode surface from electrolyte components, among them triiodide,and also from water, the presence of traces of the latter in a DSSCbeing hardly avoidable.

The terminal group of the compacting compound may be an anionic group.Such terminal group may be selected among the same group as theanchoring groups, that is to say SO₃ ⁻, CO₂ ⁻, PO²⁻ ₃, PO₃H⁻, CONHO⁻.The terminal group of the compacting compound may be a cationic group.Such terminal group may be selected among ammonium, phosphonium,sulfonium, imidazolium, pyrrolidonium and pyridinium groups.

In turn, when the molecules co-adsorbed with the sensitizing dye have asufficient chain length and if the ends of these chains bear a chargedgroup (Y), these groups surmount the hydrophobic level of the mono-layerand are capable of repelling species present in the electrolyte, therebypreventing once again direct interaction of the species of theelectrolyte with parts of the semiconductive metal oxide surface itself.

In view of an outdoor use, exposed to sun at elevated temperatures, thecompacting compound is preferably selected so that said self-assembledmonolayer is a dense packed monolayer having an order-disordertransition temperature above 80° C.

Preferred compacting compounds are selected among compounds of followingformulae (1) to (27)

With the proviso that P=Q=H (hydrogen)

-   -   or P=H and Q=F (fluoride)    -   or P=Q=F    -   that X and X′ are, independently one from the other, one of the        groups SO₃ ⁻, CO₂ ⁻, PO₃ ²⁻, PO₃H⁻ and CONHO⁻    -   that n, n′ and n″ designate the same or different integers from        1 to 20    -   that Y and Y′ are, independently one from the other, one of the        groups SO₃ ⁻, CO₂ ⁻, PO₃ ²⁻, PO₃H⁻ and CONHO⁻ or a group having        one of formulae (101) to (106)

wherein R₁, R₂, R₃ designate independently one from the other H, aphenyl group or an alkyl group of 1 to 20 carbon atoms.

In particular, the compacting compound may be selected from the groupconsisting of alkyl carboxylic acids, alkyl dicarboxylic acids, alkylcarboxylates, alkyl phosphonic acids, alkyl phosphonates, alkyldiphosphonic acids, alkyl diphosphonates, alkyl sulphonic acids, alkylsulphonates, alkyl hydroxamic acids, alkyl hydroxamates, wherein alkylis linear or branched from C₁ to C₂₀, derivatives of said alkylhydroxamic acids bearing a terminal group Y of one of formulae (101) to(106) or an anionic terminal group as aforesaid, cyclohexane-carboxylicacid, adamentane acetic acid, adamentane propionic acid and4-pentylbicyclo(2,2,2)-octane-1-carboxylic acid.

None of the above-cited compacting compounds are electron donatingspecies.

The chain length of the compacting compound, i.e. the length of thehydrophobic portion, is adapted to the dimension of the dye molecule, inparticular to the length of substituent R, i.e. ΠRal)_(q).

According to another aspect of the DSSC, object of the presentinvention, the electrolyte of the DSSC may comprise a polar organicsolvent having a high boiling point. Boiling points over 100° C. atstandard atmospheric pressure are preferred. A suitable compound to beused as organic solvent in the framework of the present invention may befound among nitrites. A preferred nitrile is 3-methoxypropionitrile(MPN). The solvent may be useful on one hand for solubilizing anelectrochemically active salt present in the electrolyte, and/or thecompound forming the redox couple with an ion of said salt.

In still another aspect of the DSSC according to the present invention,the electrolyte may comprise, instead of an electrochemically activesalt which is solid at ambient temperature and shall be dissolved in asolvent, a so-called “room temperature molten salt”, anelectrochemically active salt having a melting point lower than ambienttemperature, or a salt selected so that the mixture formed by this saltand another species of the redox system has a melting point lower thanambient temperature. Then, presence of a solvent may be avoided. Thecation of the electrochemically active salt may comprise at least onequaternary nitrogen. The quaternary nitrogen may be comprised in a groupselected from imidazolium and triazolium type groups, corresponding tothe following general formulae (a) or (b):

where the groups R₁, R₂, R₃, R₄ and R₅ are identical or different andare selected from hydrogen and linear or branched alkyl groups, with 1to 20 carbon atoms, linear or branched alkoxy groups with 1 to 20 atomsof carbon, fluoride substitution derivatives of alkyl groups, alkenylgroups, and combinations of these groups and the correspondinghalogenides, or from the alkoxyalkyl and polyether groups.

The cation of the electrochemically active salt may also be an ammonium,a phosphonium or a sulfonium group corresponding to the followinggeneral formulae (c), (d) or (e):

In which groups R₁, R₂, R₃, R₄ have the same meanings as above.

The anion of said ionic liquid salt may be selected from halide ions, ora polyhalide ion, or a complex anion containing at least one halide ion,CF₃SO₃ ⁻, or CF₃COO⁻ or (CF₃SO₂)₃C⁻ or NO₃ ⁻ or PF₆ ⁻ or BF₄ ⁻ or N(CN)₂⁻ or NCS⁻ SeCN⁻ or ClO₄ ⁻ or C(CN)₃ ⁻ or R₆SO₃ ⁻ or R₆SO₄ ⁻, where R₆ isselected from hydrogen and linear or branched alkyl groups, with 1 to 20carbon atoms, linear, or branched alkoxy groups with 1 to 20 atoms ofcarbon.

The redox system of the electrolyte may comprise two salts or more, eachhaving a melting point below ambient temperature, the anions forming acouple of two different electrolytes, for example the iodide/bromidecouple.

In a still further aspect of the DSSC, object of the present invention,the electrolyte incorporates a first compound co-operating with eitherthe anion or the cation of the electrochemically active salt, that is tosay forming a redox couple with said ion. As a well-known example ofsuch a couple, if the anion of the electrochemically salt is I⁻, theneutral molecule, respectively element, is iodine.

In still a further aspect of the DSSC, object of the present invention,the electrolyte may incorporate a stabilizing additive in form of aneutral molecule comprising one or more nitrogen atom(s) with a loneelectron pair.

Said neutral molecule may be selected from molecules having followingformula:

wherein R′₁ and R′₂ can be H, alkyl, alkoxyl, alkenyl, alkynyl,alkoxy-alkyl, poly-ether, and/or phenyl, independently one from theother, the number of carbon atoms of each substituent ranging from 1 to20, the substitute being linear or branched.

Preferred compounds are Benzimidazole, 1-methylbenzimidazole,1-methyl-2-phenyl benzimidazole and 1,2 dimethyl benzimidazole.

The presence of the afore-said neutral additive compound in theelectrolyte increases the stability of the DSSC.

Other particulars and advantages of the DSSC according to the invention,in particular improved performance and stability at high temperature,will appear to those skilled in the art from the description of thefollowing examples in connection with the drawings, which show:

FIG. 1: synthetic route of donating ligands L1;

FIG. 2: synthetic route of RuLL1(NCS)₂;

FIG. 3: absorption spectra of Z910, N-719, and Z-907 anchored on a 8 μmthickness transparent nanocrystalline TiO₂ film;

FIG. 4: photocurrent action spectrum of device A sensitised with Z910dye;

FIG. 5: current density-voltage characteristics of devices A with Z910dye under AM 1.5 sunlight (100 mW cm⁻²) illumination and in the dark.Cell active area: 0.158 cm². Outside of the active area is completelymasked with black plastic to avoid the diffusive light;

FIG. 6: detailed photovoltaic parameters of devices B with Z910 dyeduring successive one sun visible-light soaking at 55° C.;

FIG. 7: detailed photovoltaic parameters of devices C with K19 dye at80° C.;

FIG. 8: detailed photovoltaic parameters of devices C with K19 dyeduring successive one sun visible-light soaking at 55° C.;

FIG. 9: detailed photovoltaic parameters of devices E with K19 dye and1-decylphosphonic acid as coadsorbent at 80° C.;

FIG. 10: detailed photovoltaic parameters of devices E with K19 dye and1-decylphosphonic acid as coadsorbent during successive one sunvisible-light soaking at 55° C.;

FIGS. 11 and 12: the molecular structures of examples of ligands L1;

FIG. 13: the molecular structures of K 60 and Z910.

EXAMPLE I Synthesis of Ligands L1

The synthesis steps are shown in FIG. 1.

1) 4-Hexyloxybenzaldehyde, Intermediate Compound 1

4-formyl-phenol (5 g, 41 mmol), iodohexane (10.5 g, 49 mmol) and K₂CO₃(8.5 g, 61 mmol) in acetonitrile (150 ml) were refluxed overnight underN₂. After being cooled to room temperature, water (10 ml) was added andacetonitrile was evaporated. Water (150 ml) and Et₂O (150 ml) were thenadded. The ethereal layer was extracted and washed with water (2×100ml), brine (100 ml), dried over MgSO₄, filtered and evaporated todryness to afford 8.3 g (98%) of compound 1 as a slightly yellow oilafter during at 80° C. under vacuum.

¹H-NMR (CDCl₃, 298K, 200 MHz, δ ppm) 0.94 (t, J=6.5 Hz, 3H), 1.3-1.6 (m,6H), 1.80 (m, 2H), 4.05 (t, J=6.5 Hz, 2H), 7.00 (d, J=8.7 Hz, 2H), 7.83(d, J=8.7 Hz, 2H), 9.89 (s, 1H). ¹³C-NMR (CDCl₃, 298K, 50 MHz, δ ppm)14.0, 22.5, 25.6, 29.0, 31.5, 68.4, 114.7, 129.7, 131.9, 164.2, 190.7.

2) 1-Hexyloxynaphthalene, Intermediate Compound 2

1-naphthol (5 g, 34.7 mmol), iodohexane (8.82 g, 41.6 mmol) and K₂CO₃(7.2 g, 50 mmol) in acetonitrile (150 ml) were refluxed overnight underN₂. After being cooled to room temperature, water (10 ml) was added andacetonitrile was evaporated. Water (150 ml) and Et₂O (150 ml) wereadded. The ethereal layer was extracted and washed with water (2×100ml), brine (100 ml), dried over MgSO₄, filtered and evaporated todryness to afford 7.8 g (98%) of compound 2 as an orange oil afterduring at 80° C. under vacuum.

¹H-NMR (CDCl₃, 298K, 200 MHz, δ ppm) 0.99 (t, J=6.5 Hz, 3H), 1.3-1.7 (m,6H), 1.96 (m, 2H), 4.18 (t, J=6.5 Hz, 2H), 6.84 (dd, J=1.7 and 6.8 Hz,1H), 7.4-7.6 (m, 4H), 7.84 (m, 1H), 8.35 (m, 1H). ¹³C-NMR (CDCl₃, 298K,50 MHz, δ ppm) 14.1, 22.7, 26.0, 29.3, 31.7, 68.1, 104.5, 119.9, 122.1,125.0, 125.8, 125.9, 126.3, 127.4, 134.5, 154.9.

3) 1-Hexyloxy-4-formylnaphthalene, Intermediate Compound 3

POCl₃ (3.22 g, 21 mmol) was dropwise added to a solution of compound 2(4 g, 17.5 mmol) in anh. DMF (5 ml) at room temperature and under N₂.The resulting dark red solution was heated to 100° C. for 3 hours.Concentrated AcONa solution (5 ml) was then added and heating wascontinued for 2 hours more. After being cooled to room temperature,water (100 ml) was added. The mixture was extracted with Et₂O (2×150ml), the ethereal combined fractions were washed with 10% HCl solution(100 ml), water (100 ml), dried over MgSO₄, filtered and evaporated todryness. Recrystallisation of the brown solid from EtOH afford 2.1 g(47%) of compound 3 as brownish crystals.

¹H-NMR (CDCl₃, 298K, 200 MHz, δ ppm) 0.95 (t, J=7 Hz, 3H), 1.3-1.7 (m,6H), 1.98 (m, 2H), 4.24 (t, J=6.4 Hz, 2H), 6.90 (d, J=8 Hz, 1H), 7.58(t, J=8 Hz, 1H), 7.71 (t, J=8 Hz, 1H), 7.90 (d, J=8 Hz, 1H), 8.37 (d,J=8 Hz, 1H), 9.32 (d, J=8 Hz, 1H), 9.89 (s, 1H). ¹³C-NMR (CDCl₃, 298K,50 MHz, δ ppm) 14.0, 22.6, 25.8, 29.0, 31.5, 68.8, 103.5, 122.4, 124.7,124.8, 125.6, 126.3, 129.4, 131.9, 139.8, 160.4, 192.2.

4) 4,4′-bis[(trimethylsilyl)methyl]-2,2′-bipyridine, IntermediateCompound 4, was Synthesized According to Procedure Published by A. P.Smith, J. J. S. Lamba, and C. L. Fraser (Organic Synthesis, 78, pp.82-90).5) 4,4′-bis(chloromethyl)-2,2′-bipyridine, Intermediate Compound 5

A solution composed of compound 4 (2 g, 6.1 mmol), hexachloroethane (5.8g, 24.3 mmol) and KF (1.42 g, 24.3 mmol) in anhydrous DMF (30 ml) wasstirred overnight at room temperature under N₂. EtOAc (150 ml) wasadded. The organic layer was washed with water (5×100 ml), dried overMgSO₄ and evaporated to dryness. The resulting solid was dissolved inthe minimum volume of hexane and let stand in the freezer for few hours.The resulting white crystalline solid was filtered and washed with smallcold portions of hexane to afford 1.4 g (91%) of compound 5 as whitecrystalline solid.

¹H-NMR (CDCl₃, 298K, 200 MHz, δ ppm) 4.65 (s, 4H), 7.40 (d, J=5 Hz, 2H),8.45 (s, 2H), 8.70 (d, J=5 Hz, 2H).

6) 4,4′-bis(diethyl methylphosphonate)-2,2′-bipyridine, IntermediateCompound 6.

Compound 5 (2.6 g, 10.3 mmol) was refluxed overnight under N₂ intriethylphosphite (50 ml). Excess P(OEt)₃ was evaporated and theresulting brown oil was column chromatographed (Al₂O₃, CH₂Cl₂/MeOH:98/2). The yellow oil thus obtained was dissolved in a mixture ofCH₂Cl₂/hexane (1/50 ml) and let stand in the freezer to afford afterfiltration 4 g (85%) of compound 6 as a slightly yellow crystallinesolid.

¹H-NMR (CDCl₃, 298K, 200 MHz, δ ppm) 1.27 (t, J=7 Hz, 12H), 3.18 (s,2H), 3.29 (s, 2H), 4.08 (m, 8H), 7.33 (d, J=5 Hz, 2H), 8.34 (s, 2H),8.60 (d, J=5 Hz, 2H).

7) Procedure for the Synthesis of4,4′-bis(4-hexyloxystyryl)-2,2′-bipyridine, Compound 7, and4,4′-bis(4-hexyloxynaphthalene-1-vinyl)-2,2′-bipyridine, Compound 8.

Solid ^(t)BuOK (740 mg, 6.6 mmol) was added to an anh. DMF (50 ml)solution of compound 6 (1 g, 2.2 mmol) and compound 1 or 3 (2.52 g, 5.5mmol) and the resulting mixture was stirred overnight at roomtemperature under N₂. A copious precipitate appeared after few minutes.DMF was evaporated and the resulting slurry was stirred 30 minutes inMeOH (100 ml). The white precipitate was filtered, washed with smallportions of MeOH and dried to afford compound 7 (85%) as a slightly pinksolid or compound 8 (78%) as a white solid.

7: ¹H-NMR (CDCl₃, 298K, 200 MHz, δ ppm) 0.93 (t, J=6.3 Hz, 6H), 1.2-1.5(m, 12H), 1.85 (m, 4H), 4.00 (t, J=6.4 Hz, 4H), 6.93 (d, J=8.7 Hz, 4H),7.00 (d, J=17 Hz, 2H), 7.38 (d, J=5 Hz, 2H), 7.43 (d, J=17 Hz, 2H), 7.51(d, J=8.7 Hz, 4H), 8.53 (s, 2H), 8.66 (d, J=5 Hz, 2H). ¹³C-NMR (CDCl₃,298K, 50 MHz, δ ppm) 14.0, 22.6, 25.7, 29.2, 31.6, 68.1, 114.8, 118.0,120.8, 123.8, 128.4, 128.8, 133.0, 146.1, 149.4, 156.5, 159.7.

8: ¹H-NMR (CDCl₃, 298K, 200 MHz, δ ppm)) 0.96 (t, J=7 Hz, 6H), 1.3-1.7(m, 12H), 1.98 (m, 4H), 4.21 (t, J=6.4 Hz, 4H), 6.88 (d, J=8 Hz, 2H),7.13 (d, J=16 Hz, 2H), 7.50-7.67 (m, 6H), 7.75 (d, J=8 Hz, 2H), 8.20 (d,J=16 Hz, 2H), 8.22-8.27 (m, 2H), 8.36-8.41 (m, 2H), 8.64 (s, 2H), 8.73(d, J=5 Hz, 2H). ¹³C-NMR (CDCl₃, 298K, 50 MHz, δ ppm) 14.4, 22.6, 25.9,29.2, 31.6, 68.3, 104.6, 118.4, 120.9, 122.7, 123.3, 124.8, 125.2,125.7, 126.1, 126.9, 127.0, 130.4, 132.3, 146.3, 149.5, 155.7, 156.6.

10) 4,4′-bis[4-(1,4,7,10-Tetraoxyundecyl)styryl]-2,2′-bipyridine.

3,6,9-Trioxydecyl 4-toluenesulfonate and4-(1,4,7,10-Tetraoxyundecyl)benzaldehyde were synthesized according toreference C. Lottner, K.-C. Bart, G. Bernhardt, H. Brunner J. Med. Chem.2002, 45, 2079-2089).

Solid tBuOK (1.5 g, 13.4 mmol) was added to a solution of4,4′-bis(diethylmethylphosphonate)-2,2′-bipyridine (1.5 g, 3.3 mmol) and4-(1,4,7,10-Tetraoxyundecyl)benzaldehyde (2.1 g, 7.8 mmol) in anhydrousDMF (80 ml). The resulting mixture was stirred overnight at roomtemperature under nitrogen. After evaporation of DMF, water (100 ml) wasadded and extracted with CH₂Cl₂ (3×150 ml). The combined organicfractions were washed with water (100 ml), brine (100 ml), dried overMgSO₄, filtered and evaporated to dryness. The resulting residue wasthen dissolved in the minimum volume of CH₂Cl₂ and precipitated byaddition of Et₂O with rapid stirring. The solid was filtered, washedwith Et₂O and dried to afford 1.5 g (66%) of the titled compound as aslightly beige solid.

¹H NMR (200 MHz, 25° C., CDCl₃) δ 3.39 (s, 6H), 3.5-3.7 (m, 16H), 3.88(t, J=4.5 Hz, 4H), 4.18 (t, J=4.5 Hz, 4H), 6.94 (d, J=8 Hz, 4H), 7.00(d, J=16 Hz, 2H), 7.4-7.5 (m, 8H), 8.52 (s, 2H), 8.65 (d, J=5 Hz, 2H).¹³C NMR (50 MHz, 25° C., CDCl₃) δ 59.0, 67.5, 69.7, 70.6, 70.7, 70.8,71.9, 114.9, 118.0, 120.8, 124.0, 128.4, 129.2, 132.8, 146.1, 149.4,156.5, 159.3.

Those skilled in the art will observe that if the symmetric startingcompound of step 4, e.g. 4,4′-bis(methyl)-2,2′-bipyridine is replaced byan asymmetric starting compound, e.g. 4-methyl-2,2′-bipyridine, anasymmetric compound L1, e.g. 4-(4-hexyloxystyryl)-2,2′ bipyridine willbe synthesized.

EXAMPLE II Synthesis of Z910

The synthesis of Z910 was performed according to a one pot synthesismethod reported in Nat. Mater (2003) 2, 402. [RuCl₂(p-cymene)]₂ (0.15 g,0.245 mmol) was dissolved in DMF (50 ml) and to this solution dmsbpy(0.206 g, 0.49 mmol) was added. The reaction mixture was heated to 60°C. under nitrogen for 4 h with constant stirring. To this reaction flaskH₂dcbpy (0.12 g, 0.49 mmol) was added and refluxed for 4 h. Finally,excess of NH₄NCS (13 mmol) was added to the reaction mixture andcontinued the reflux for another 4 h. The reaction mixture was cooleddown to room temperature and the solvent was removed by usingrotary-evaporator under vacuum. Water was added to the flask and theinsoluble solid was collected on a sintered glass crucible by suctionfiltration. The crude was dissolved in a basic methanol solution andpurified by passing through a Sephadex LH-20 column with methanol as aneluent. After collecting main band and evaporating the solvent, theresultant solid was redissolved in water. Lowering the pH to 3.1 bytitration with dilute nitric acid produced Z910 as a precipitate. Thefinal product was washed thoroughly with water and dried under vacuum.¹H NMR (δ_(H)/ppm in CD₃OD+ NaOD) 9.4 (d, 1H), 9.2 (d, 1H), 8.9 (s, 1H),8.8 (s, 1H), 8.3 (s, 1H), 8.15 (s, 1H), 7.9 (d, 1H), 7.80 (d, 1H), 7.7to 6.9 (m, 16H), 4.1 (s, 3H), 4.0 (s, 3H). Anal. Calc. forRuC₄₂H₃₄N₆O₇S₂: C, 56.0; H, 3.78; N, 9.34%. Found: C, 55.22; H, 3.97; N,9.39%.

The molecular structure of Z910 is given in FIG. 13.

FIG. 3 compares the electronic absorption spectra of 8 ┌m mesoporousTiO₂ films grafted respectively with Z907, N719 and Z910 dyes.

By extending the π-conjugated system of the ligand, the metal-to-ligandcharge transfer (MLCT) transitions are red shifted with higher molarextinction coefficient. In addition to the increase in the opticaldensity of MLCT transitions there is a huge increase in the opticaldensity of ligand to ligand charge transitions in the UV regions. TheseUV transitions can serve as UV filters in DSSCs

EXAMPLE III

The Synthesis Steps of K19 or K24 or K60 are Shown in FIG. 2.

Synthesis of Dye K19 Ru(LL1) (NCS)₂

Compound 7 (200 mg, 0.36 mmol) and dichloro(p-cymene)ruthenium(II) dimer(109 mg, 0.18 mmol) were refluxed in argon degased EtOH (50 ml) for 4hours under argon. The orange-brown solution was evaporated to drynessto afford quantitatively the intermediate complex RuL(p-cymene)Cl₂ as abrown solid. This complex and 4,4′-dicarboxy-2,2′-bipyridine (88 mg,0.36 mmol) were heated to 140° C. in degased anh. DMF for 4 hours underargon and in the dark. To the resulting dark green solution was addedsolid NH₄NCS (411 mg, 5.4 mmol) and the mixture was allowed to heat 4hours more at 140° C. under argon and in the dark. DMF was evaporatedand water (200 ml) was added. The formed purple solid was filtered off,washed with water, Et₂O, and purified on LH-20 sephadex to affordcomplex K₁₉. ¹H NMR (δ_(H)/ppm in CD₃OD+ NaOD) 9.4 (d, 1H), 9.2 (d, 1H),8.9 (s, 1H), 8.8 (s, 1H), 8.3 (s, 1H), 8.15 (s, 1H), 8.0 (d, 1H), 7.80(d, 1H), 7.7 to 6.9 (m, 16H), 4.1 (s, 3H), 1.8 (t, 2H), 1.6 to 1.4 (m,8H), 1.0 (t, 3H). The molecular structure of Ru(dcbpy) (dmsbpy) (NCS)₂(where dcbpy is 4,4′-dicarboxylic acid-2,2′-bipyridine) referred to K19is shown in FIG. 2.

EXAMPLE IV Synthesis of Dye K24 Ru (LL1) (NCS)₂

Compound 8 (300 mg, 0.45 mmol) and dichloro(p-cymene)ruthenium(II) dimer(139 mg, 0.227 mmol) were refluxed in argon degased EtOH (50 ml) for 4hours under argon. The orange-brown solution was evaporated to drynessto afford quantitatively the intermediate complex RuL(p-cymene)Cl₂ as abrown solid. This complex and 4,4′-dicarboxy-2,2′-bipyridine (111 mg,0.45 mmol) were heated to 140° C. in degased anh. DMF for 4 hours underargon and in the dark. To the resulting dark green solution was addedsolid NH₄NCS (520 mg, 6.8 mmol) and the mixture was allowed to heat 4hours more at 140° C. under argon and in the dark. DMF was evaporatedand water (200 ml) was added. The formed purple solid was filtered off,washed with water, Et₂O, and purified on LH-20 sephadex to affordcomplexes K24.

EXAMPLE V Synthesis of Dye K 60: Ru(LL′) (NCS)₂

Compound 10 (850 mg, 1.24 mmol) and dichloro(p-cymene)ruthenium(II)dimer (380 mg, 6.2 mmol) were refluxed in argon degased EtOH (50 ml) for4 hours under argon. The orange-brown solution was evaporated to drynessto afford quantitatively the intermediate complex RuL(p-cymene)Cl₂ as abrown solid. This complex and 4,4′-dicarboxy-2,2′-bipyridine (303 mg,1.24 mmol) were heated to 140° C. in degased anh. DMF for 4 hours underargon and in the dark. To the resulting dark green solution was addedexcess solid NH₄NCS (1.5 g) and the mixture was allowed to heat 4 hoursmore at 140° C. under argon and in the dark. DMF was evaporated andwater (200 ml) was added. The formed purple solid was filtered off,washed with water, Et₂O, and purified on LH-20 sephadex to affordcomplex K 60 as purple solid. The molecular structure of Ru (dcbpy)(4,4′-bis[4-(1,4,7,10-Tetraoxyundecyl)styryl]-2,2′-bipyridine (NCS)₂(where dcbpy is 4,4′-dicarboxylic acid-2,2′-bipyridine) referred to K 60is shown in FIG. 13.

EXAMPLE VI Fabrication and Photovoltaic Performance of Z910 SensitizedSolar Cells

A screen-printed double layer of TiO₂ particles was used as photoanode.A 10 μm thick film of 20 nm sized TiO₂ particles was first printed onthe fluorine-doped SnO₂ conducting glass electrode and further coated by4 μm thick second layer of 400 nm sized light scattering anataseparticles. Fabrication procedure for the nanocrystalline TiO₂photoanodes and the assembly as well as photoelectrochemicalcharacterization of complete, hot-melt sealed cells has been describedby P. Wang et al. (J. Phys. Chem. B, 2003, 107, 14336-14341). Theelectrolyte used for device A contained 0.6 M1-propyl-3-methylimidazolium iodide (PMII), 30 mM M I₂, 0.13 Mguanidinium thiocyanate, and 0.5 M 4-tert-butylpyridine in the 1:1volume mixture of acetonitrile and valeronitrile. The TiO₂ electrodeswere immersed at room temperature for 12 h into a solution containing300 μM Z910 and 300 μM chenodeoxycholic acid in acetonitrile andtert-butanol (volume ratio: 1:1). For stability tests, the electrolytewas composed of 0.6 M PMII, 0.1 M I₂, and 0.5 M N-methylbenzimidazole in3-methoxypropionitrile and the corresponding device with the Z910 dyealone is denoted as device B.

The photocurrent action spectrum of device A with Z910 as sensitizer isshown in FIG. 4. The incident photon to current conversion efficiency(IPCE) exceeds 80% in a spectral range from 470 to 620 nm, reaching itsmaximum of 87% at 520 nm. Considering the light absorption andscattering loss by the conducting glass, the maximum efficiency forabsorbed photon to current conversion efficiency is practically unityover this spectral range. From the overlap integral of this curve withthe standard global AM 1.5 solar emission spectrum, a short-circuitphotocurrent density (J_(sc)) of 17.2 mA cm^(□2) is calculated, which isin excellent agreement with the measured photocurrent. As shown in FIG.5, its short-circuit photocurrent density (J_(sc)), open-circuitphotovoltage (V_(oc)), and fill factor (ff) of device A with Z910 dyeunder AM 1.5 full sunlight are 17.2 mA cm^(□2), 777 mV, and 0.764,respectively, yielding an overall conversion efficiency (|) of 10.2%. Atvarious lower incident light intensities, overall power conversionefficiencies are also over 10.2%. With the double layer film (totalthickness of 14 μm) and electrolyte used here, the power conversionefficiencies of N-719 and Z-907 dyes are 7% less efficient than Z910.

The above-mentioned 3-methoxypropionitrile based electrolyte was usedfor the stability test of sensitizer Z910 under moderate thermal stressand visible light soaking. The advantage of using 3-methoxypropionitrilelies in its high boiling point, low volatility, non-toxicity and goodphotochemical stability, making it viable for practical application.Photovoltaic parameters (Jsc, Voc, ff, and |) of device B are 14.8 mAcm^(□2), 696 mV, 0.695, and 7.2%, respectively. The cells covered with a50 μm thick of polyester film (Preservation Equipment Ltd, UK) as a UVcut-off filter (up to 400 nm) were irradiated at open circuit under aSuntest CPS plus lamp (ATLAS GmbH, 100 mW cm⁻², 55° C.). As shown inFIG. 6, all parameters of the device are rather stable during 1000 haccelerating tests. It should be noted that under this condition thesensitizer showed similar stability but higher efficiency compared withZ-907 dye.

EXAMPLE VII Fabrication and Photovoltaic Performance of K19 SensitizedSolar Cells with a Organic Solvent Based Electrolyte

A screen-printed double layer of TiO₂ particles was used as photoanode.A 10 μm thick film of 20 nm sized TiO₂ particles was first printed onthe fluorine-doped SnO₂ conducting glass electrode and further coated by4 μm thick second layer of 400 nm sized light scattering anataseparticles. Fabrication procedure for the nanocrystalline TiO₂photoanodes and the assembly as well as photoelectrochemicalcharacterization of complete, hot-melt sealed cells C has been describedabove. The electrolyte used for device C contained 0.6 M1,2-dimethyl-3-propylimidazolium iodide (DMPII), 0.1 mM M I₂, and 0.5 μMN-methylbenzimidazole in 3-methoxypropionitrile. The TiO₂ electrodeswere immersed at room temperature for 12 h into a solution containing300 μM K19 in the mixture of acetonitrile and tert-butanol (volumeratio: 1:1).

FIG. 7 shows the evolution of photovoltaic parameters of device C at 80°C. in the dark. FIG. 8 shows the evolution of photovoltaic parameters ofdevice C covered with a UV filter at 55-60° C. under AM 1.5 sunlight(100 mW/cm²).

EXAMPLE VIII Fabrication and Photovoltaic Performance of K19 SensitizedSolar Cells with a Organic Solvent Based Electrolyte

A screen-printed double layer of TiO₂ particles was used as photoanode.A 10 μm thick film of 20 nm sized TiO₂ particles was first printed onthe fluorine-doped SnO₂ conducting glass electrode and further coated by4 μm thick second layer of 400 nm sized light scattering anataseparticles. Fabrication procedure for the nanocrystalline TiO₂photoanodes and the assembly as well as photoelectrochemicalcharacterization of complete, hot-melt sealed cells has been describedas above. The electrolyte used for device D contained 0.2 M I₂, and 0.5M N-methylbenzimidazole in the 65/35 volume mixture of1-propyl-3-methylimidazolium iodide (PMII) and1-ethyl-2-methylimidazolium tricyanomethide [EMIC(CN)₃). The TiO₂electrodes were immersed at room temperature for 12 h into a solutioncontaining 300 μM K₁₉ in the mixture of acetonitrile and tert-butanol(volume ratio: 1:1). Table 1 gives the detailed photovoltaic paprametersof device D under illumination of different light intensities.

TABLE 1 Detailed photovoltaic parameters of device D. P_(in)/mW cm⁻²J_(sc)/mA cm⁻² V_(oc)/mV P_(max)/mW cm⁻² ff η/% 9.45 1.42 634 0.7 0.787.4 51.7 7.31 682 3.75 0.77 7.2 99.9 13.0 700 6.7 0.74 6.7

The spectral distribution of the lamp simulates air mass 1.5 solarlight. Incident power intensity: P_(in); Short-circuit photocurrentdensity: J_(sc); Open-circuit photovoltage: V_(oc); Maximum electricityoutput power density: P_(max); Fill factor: ff=P_(max)/P_(in); Totalpower conversion efficiency: η. Cell active area: 0.158 cm².

EXAMPLE IX Fabrication and Photovoltaic Performance of Cells with a TiO₂Film Cografted with K19 Dye and 1-decylphosphonic Acid Coadsorbent

A screen-printed double layer of TiO₂ particles was used as photoanode.A 10 μm thick film of 20 nm sized TiO₂ particles was first printed onthe fluorine-doped SnO₂ conducting glass electrode and further coated by4 μm thick second layer of 400 nm sized light scattering anataseparticles. Fabrication procedure for the nanocrystalline TiO₂photoanodes and the assembly as well as photoelectrochemicalcharacterization of complete, hot-melt sealed cells has been describedabove. The electrolyte used for device E contained 0.6 M1,2-dimethyl-3-propylimidazolium iodide (DMPII), 0.1 mM M I₂, and 0.5 MN-methylbenzimidazole in 3-methoxypropionitrile. The TiO₂ electrodeswere immersed at room temperature for 12 h into a solution containing300 μM K₁₉ dye and 75 μM 1-decylphosphonic acid coadsorbent in themixture of acetonitrile and tert-butanol (volume ratio: 1:1).

FIG. 9 shows the evolution of photovoltaic parameters of device E at 80°C. in the dark. It is clear that the presence of 1-decylphosphonics hasenhanced the stability of photovoltage under the thermal stress at 80°C. FIG. 10 shows the evolution of photovoltaic parameters of device Ecovered with a UV filter at 55-60° C. under AM 1.5 sunlight (100mW/cm²).

In conclusion, new heteroleptic polypyridyl ruthenium complexes withhigh molar extinction coefficients have been synthesized anddemonstrated as highly efficient, stable sensitizers for nanocrystallinesolar cells. Enhancing the molar extinction coefficient of sensitizershas been demonstrated to be an elegant strategy to improve thephotovoltaic performance of dye sensitized solar cells.

1. An organometallic complex of a metal Me selected from the groupconsisting of Ru, Os and Fe, comprising as a ligand a compound L1, saidcomplex being of formulaMe L1 L(Z)₂  (I) if L1 is a compound of formula (a), (b), (c), (d), (g),(h), (i) or (j)

and of formulaMe L1 L Z  (II) if L1 is a compound of formula (e) or (f)

wherein L is a ligand selected from the group of ligands of formula

wherein A and A′ are anchoring groups selected from COOH, PO₃H₂, PO₄H₂,SO₃H₂, SO₄H₂, CONHOH, deprotonated forms thereof and chelating groupswith π conducting character, wherein Z is selected from the groupconsisting of H₂O, Cl, Br, CN, NCO, NCS and NCSe and wherein at leastone of substituents —R, —R₁, —R₂, —R₃, —R′, —R₁′, —R₂′, —R₃′, —R″comprises a π system in conjugated relationship with the π system of thebidentate, respectively the tridentate structure of formulae (a) to (j),and wherein the other one(s) of substituents —R, —R₁, —R₂, —R₃, —R′,—R₁′, —R₂′, —R₃′, —R″ is (are) the same or a different substituentincluding a π system, or is (are) selected from H, OH, R2, (OR2)_(n),N(R2)₂, where R2 is an alkyl of 1-20 carbon atoms or linear R cyclicpolyether, 0<n<5.
 2. An organometallic complex as claimed in claim 1,said complex being of formulaMe L1 L(Z)₂  (I) wherein Me designates Ru, Os or Fe, wherein L isselected from ligands

wherein Z is selected from H₂O, —Cl, —Br, —I, —CN, —NCO, —NCS and —NCSe.wherein L1 is a substituted bipyridine of formula

wherein —R, —R₁, —R₂, —R₃, —R′, —R₁′, —R₂′, —R₃′, —R″ is a substituentselected from the group of substituents (1), (2) and (3)

wherein p is an integer from 1 to 4 or is 0 wherein q is an integer from1 to 4 wherein Rar is a monocyclic or polycyclic aryl from C6 to C22wherein each —Ral is, independently one from the others, —H, —R1,—(O—R1)_(n), —NHR1 or —N(R1)₂,

wherein R1, R′1 is an alkyl from 1 to 10 carbon atoms, 20≧X≧0, and5≧n≧0, 8≧Y≧1, Z=1 or
 2. 3. An organometallic complex as claimed in claim2, said complex being of formulaMe L1 L(Z)₂  (I) wherein Me designates Ru, Os or Fe, wherein L isselected from ligands

wherein Z is selected from H₂O, —Cl, —Br, —I, —CN, —NCO, —NCS and —NCSe.wherein L1 is a 4,4′-substituted bipyridine of formula

wherein R is a substituent selected from the group of substituents (1),(2) and (3) and R′ has the same meaning as above.

wherein p is an integer from 1 to 4 or is 0 wherein q is an integer from1 to 4 wherein Rar is a monocyclic or polycyclic aryl from C6 to C22wherein each —Ral is, independently one from the others, —H, —R1,—(O—R1)_(n), —NHR1 or —N(R1)₂,

wherein R1, R′1 is an alkyl from 1 to 10 carbon atoms, 20≧X≧0, and5≧n≧0, 8≧Y≧1, Z=1 or
 2. 4. An organometallic complex as claimed in claim3, of formula cis(NCS)₂ RuLL1, wherein L1 is of formula (a′), wherein Ris of formula (1), (2) or (3), wherein p=1, wherein Rar is selected fromthe group consisting of benzene and naphthalene, wherein q=1 to 4,wherein Ral is OR1 and wherein R1 is an alkyl of 1 to 10 carbon atoms orlinear R cyclic polyethers. 5.Cis-dithiocyanato-(2,2′-bipyridyl-4,4′-dicarboxylate)-[4,4′-bis(4-hexyloxystyryl)-2,2′bipyridyl]-Ru(II). 6.Cis-dithiocyanato-(2,2′-bipyridyl-4,4′-dicarboxylate)-[4,4′-bis(4-hexyloxynaphtalene-1-vinyl)-2,2′bipyridyl]-Ru(II). 7.Cis-dithiocyanato-(2,2′-bipyridyl-4,4′-dicarboxylate)-[4,4′-bis(3-methoxystyryl)-2,2′bipyridyl]-Ru(II). 8.Cis-dithiocyanato-(2,2′-bipyridyl-4,4′-dicarboxylate)-[4,4′-bis[4-(1,4,7,10-Tetraoxyundecyl)styryl]-2,2′-bipyridine]-Ru(II).9. A regenerative photoelectrochemical cell comprising a photoanode,said photoanode comprising at least one semi-conductive metal oxidelayer on a conductive substrate, sensitized by a photosensitizing dye, acounter electrode and an electrolyte arranged between saidsemi-conductive metal oxide layer and said counter electrode,characterized in that said photosensitizing dye is an organometalliccomplex as claimed in claim
 1. 10. A cell as claimed in claim 9,characterized in that an amphiphilic compacting compound whose molecularstructure comprises at least one anchoring group, a hydrophobic portionand a terminal group is co-adsorbed with said photosensitizing dye onsaid semi-conductive metal oxide layer in a mixed monolayer.
 11. A cellas claimed in claim 10, characterized in that said compacting compoundis selected from the group consisting of alkyl carboxylic acids, alkyldicarboxylic acids, alkyl carboxylates, alkyl phosphonic acids, alkylphosphonates, alkyl diphosphonic acids, alkyl diphosphonates, alkylsulphonic acids, alkyl sulphonates, alkyl hydroxamic acids and alkylhydroxamates, wherein said alkyl is linear or branched from C1 to C20.12. A cell as claimed in claim 10 or 11, characterized in that the molarratio of said photosensitizing dye to said co-adsorbed compactingcompound is of between 10 and ½, and in that said self-assembledmonolayer is a dense packed monolayer having an order-disordertransition temperature above 80° C.
 13. A cell as claimed in claim 10,characterized in that the length of said hydrophobic chain portion ofthe compacting compound allows said terminal group to protrude above thesensitizing dye in said monolayer.
 14. A cell as claimed in claim 9,characterized in that said electrolyte comprises a redox system, thatsaid redox system comprises an electrochemically active salt and a firstcompound forming a redox couple with either the anion or the cation ofsaid electrochemically active salt, wherein said salt is a roomtemperature molten salt, said molten salt being liquid at least betweenstandard room temperature and 80° C. above said room temperature.
 15. Acell as claimed in claim 9, characterized in that said electrolytefurther comprises a polar organic solvent having a boiling point of 100°C. or greater than 100° C. at normal atmospheric pressure.
 16. A cell asclaimed in claim 15, characterized in that said solvent is a nitrileselected from 3-methoxypropionitrile 3-ethoxypropionitrile,3-butoxupropionitrile, and butyronitrile.
 17. A cell as claimed in claim9, characterized in that said electrolyte further comprises, as anadditive, a compound formed by a neutral molecule comprising one or morenitrogen atom(s) with a lone electron pair.
 18. A cell as claimed inclaim 17, characterized in that said neutral molecule has followingformula:

wherein R′₁ and R′₂ can be H, alkyl, alkenyl, alkynyl, alkoxyl,poly-ether, and/or phenyl, independently one from the other, the numberof carbon atoms of each substituent ranging from 1 to 20, thesubstituent being linear or branched. 19.4,4′-bis(4-hexyloxystyryl)-2,2′ bipyridine. 20.4,4′-bis(4-hexyloxynaphtylene-1-vinyl)-2,2′ bipyridine.
 21. A method ofpreparing a dye-sensitized solar cell, the method comprising the step ofproviding a dye comprising at least one compound L1 of formula (a),(a′), (b), (c), (d), (e), (f), (g), (h), (i), (j) as defined above,wherein at least one of —R, —R₁, —R₂, —R₃, —R′, —R₁′, —R₂′, —R₃′, —R″ isa substituent selected from the group of substituents (1), (2) and (3)

wherein p is an integer from 1 to 4 or is 0 wherein q is an integer from1 to 4 wherein Rar is a monocyclic or polycyclic aryl from C6 to C22wherein each —Ral is, independently one from the others, —H, —R1,—(O—R1)_(n), —NHR1 or —N(R1)₂,

wherein R1, R′1 is an alkyl from 1 to 10 carbon atoms, 20≧X≧0, and5≧n≧0, 8≧Y≧1, Z=1 or 2; and, preparing the solar cell comprising saiddye.